Reduced Sea Ice and its link to frequent intraseasonal cold air outbreak during the 2009-2010 abnormal winter in Japan and East Asia

In the winter of 2009/2010, Japan and the East Asian region experienced an abnormally frequent occurrence of cold air outbreaks. Although the winter averaged temperature in the Japan Honshu islands was slightly positive (+0.81C for DJF average and +0.71C for NDJFM average), repeated decline in temperature was notable from November 2009 even through April 2010 (Fig1). One explanation for this abnormal winter season is the extreme negative condition of the Northern Hemisphere Annular Mode (NAM) that persisted from December to mid-January which marks a reversal of air mass between the Arctic and mid-latitudes. However, more explanation is needed to explain why such variability was prominent in the intraseasonal timescale and especially in Japan and East Asian region. Prior to a particular case of the cold air outbreak which reached Japan on Dec. 17, an anomalous ridge formed in western Siberia which matured into a strong high pressure anomaly in the Barents sea. Cyclonic wind anomaly accompanying this high pressure anomaly created a cold air advection from the Arctic Ocean which lead up to the cold air buildup over western Siberia. The pressure anomaly subsequently shifted westward to mature into a blocking high which created a wave-train pattern downstream advecting the cold air buildup eastward toward East Asia and Japan (Fig2). Variant of such process of cold air buildup and advection was seen throughout the winter season. Intensification of high pressure anomaly over the Barents sea was concurrent with the negative anomaly in sea ice coverage and discharge of heat flux. This study suggests that there is a strong and systematic linkage between the Arctic Ocean and the cold air outbreak over East Asia and Japan which we will focus upon in our presentation. Timeseries of temperature anomaly based on 58 stations in the main islands of Japan (above) and the daily NAM index (below).

500hPa geopotential height (contours) and 850hPa temperature anomaly (shades) regressed on the Novmenber to Marcch daily temperature over the Japan region with 10, 5, 1 day lag respectively preceding the cold air outbreak.